1. Technical Field
The present invention relates to a flow rate-measuring device and, more particularly, to a flow rate-measuring device suitable for measuring an intake air flow rate of an engine such as internal combustion engine.
2. Background Art
FIG. 13 is a front view of a conventional flow rate-measuring device disclosed in the Japanese Patent Publication (unexamined) No. 313318/1996. FIG. 14 is a sectional view taken along the line XIVxe2x80x94XIV of FIG. 13. In FIGS. 13 and 14, reference numeral 10 is a flow rate-measuring device, numeral 1 is a main body of the flow rate-measuring device, numeral 2 is a circuit substrate accommodation case, numeral 3 is an electric member for measuring the flow rate, numeral 5 is a duct where fluid to be an object of flow measurement (hereinafter referred to as fluid to be measured) flows, and numeral 6 is a honeycomb. An arrow A indicates the flow direction of the fluid to be measured (this is the same in the respective drawings described below). The circuit substrate accommodation case 2 is connected with the flow rate-measuring device body 1, and the circuit substrate accommodation case 2 is attached to the duct 5 and holds the flow rate-measuring device body 1 in the duct 5. The flow rate-measuring device body 1 is comprised of a terminal holding member 13 and a flow rate-measuring passage 11 consisting of a cylinder having a bell-mouthed inlet. The electric member 3 is comprised of a flow rate-detecting element 31 consisting of a flow rate-detecting resistance 311, a temperature-compensating resistance 312, a circuit substrate 34 accommodated in the circuit substrate accommodation case 2, a terminal 35, and a connector 36. The flow rate-detecting element 31 and the temperature-compensating resistance 312 are electrically connected with the circuit substrate 34 through the terminal 35, and the circuit substrate 34 is electrically connected with the connector 36. In this manner, the flow rate-detecting element 31 and so on are operated by electric power supplied through the connector 36. The flow rate of the fluid to be measured detected by the flow rate-detecting element 31 is transformed into an electric signal, and is inputted via the circuit substrate 34 to an external receiver (not shown in the drawings). The flow rate-detecting resistance 311 is formed by putting a platinum film in the form of teeth of a comb on a ceramic substrate. The temperature-compensating resistance 312 is also formed of platinum.
In such a conventional flow rate-measuring device 10, a heating current flowing in the flow rate-detecting resistance 311 of the flow rate-detecting element 31 is controlled by a circuit (not shown in the drawings) formed in the circuit substrate 34 so that the average temperature of the flow rate-detecting resistance 311 is higher than the temperature of the fluid to be measured detected by the temperature-compensating resistance 312 by a predetermined value. The heating current supplied to the flow rate-detecting resistance 311 is detected from dependence of the resistance value of the flow rate-detecting resistance 311 on temperature and cooling effect of the flow rate-detecting resistance 311 based on the flow of the fluid to be measured, and this heating current value is inputted as a flow rate signal to the external receiver.
FIG. 15 shows a sectional view of an example of a pipe system in a case where the foregoing flowrate-measuring device 10 is used for measuring an air intake flow rate of an internal combustion engine for vehicles. In FIG. 15, numeral 6 is the honeycomb, numeral 7 is an air cleaner case, and numeral 71 is an air cleaner element arranged in the air cleaner case 7. The air cleaner element 71 is a filter composed of a nonwoven fabric or filter paper. The air cleaner element 71 catches dust in the air sucked into the internal combustion engine (not shown) to prevent dust from coming into the internal combustion engine. In the pipe system as described above, the flow rate-measuring device 10 is arranged in the downstream of the air cleaner element 71.
The air cleaner element 71 is plugged by accumulation of dust increasing with the increase of the air intake quantity due to operation of the internal combustion engine. Owing to the plugged air cleaner element 71, eddy is generated or flow velocity distribution becomes uneven in the flow of the intake air having passed through the air cleaner element 71. As a result, there is a great difference in the flow of the intake air in the upper stream of the flow rate-measuring device 10 depending upon whether it is before the air cleaner element 71 is plugged or after the element is plugged are greatly different. Even when the flow of the intake air is greatly changed, the change of the flow of the intake air is moderated by the honeycomb 6 arranged upstream from the flow rate-measuring device 10 and the bell-mouthed configuration of the flow rate-measuring passage 11 in the flow rate-measuring device 10, and consequently, value of an error in the flow rate measured by the flow rate-measuring device 10 is lowered. In this situation, the honeycomb 6 functions to remove whirl flow components such as eddy. The bell-mouthed configuration functions to contract the flow of the air flowing into the flow rate measuring passage 11 to a certain degree and reduce unevenness in the flow velocity distribution. In addition, it is certain that the cylindrical flow rate measuring passage 11 has the bell-mouthed configuration, but the function of reducing eddies is not very large. Therefore, the flow rate-measuring device 10 having the flow rate-measuring passage 11 of such a cylindrical configuration is used generally in combination with the honeycomb 6.
In recent years, under the background of increasing tendency of demanding for smaller engine rooms, the so-called plug-in type flow rate-measuring device, for example, a flow rate-measuring device disclosed in Japanese Patent Publication (unexamined) No. 219838/1996, being capable of easily attached to a duct has been proposed. However, the flow rate-measuring device disclosed in the foregoing official gazette is not provided with a special fluid passage for measuring the flow rate of the fluid to be measured like the flow rate-measuring passage 11 shown in FIG. 14 as described above. Therefore, it is difficult to attach a rectifier like the foregoing honeycomb to the flow rate-measuring device itself. This causes a problem such that the error in measuring the flow rate is increased when the air cleaner element is plugged.
In order to reduce the error in the flow rate measured by the plug-in type flow rate-measuring device, in some cases, a rectifier is attached to the air cleaner case or an intake pipe. However, in such a case, when using a rectifier with small meshes to obtain a sufficient rectification performance, pressure loss is increased, and the quantity of air possible to be taken into the internal combustion engine is limited, whereby output of the internal combustion engine is lowered. There is another problem in that the rectifier is plugged by fine dust that has passed through the air cleaner element. On the other hand, when using a rectifier with large meshes is used to solve the problem of being plugged, not only the rectification effect is lowered but further problems such as increase in thickness of a boundary layer and unevenness in friction stress occur in a flow rate detecting portion of the foregoing flow rate-measuring device due to eddy generated in the downstream of the rectifier. As a result, there arises a turbulence in the signal of a detected flow rate, and the flow rate is not detected accurately in some cases. Moreover, there is a further problem in that due to the necessity of adding any rectifier such as honeycomb to the flow rate-measuring device, cost for manufacturing the flow rate-measuring device is increased.
A flow rate-measuring device disclosed in the Japanese Patent Application No. 131570/1999 (hereinafter referred to as the prior application art) was developed by some of the inventors of the present invention in order to solve the above-discussed problems. Construction of this flow rate-measuring device is going to be described briefly with reference to FIGS. 16 and 17.
FIG. 16 is a front view of the foregoing flow rate-measuring device. FIG. 17 is a sectional view taken along the line XVIIxe2x80x94XVII of FIG. 16. In FIGS. 16 and 17, the flow rate-measuring device 10 is attached to the duct 5 through which the fluid to be measured flows, and the flow rate-measuring device 10 has the columnar flow rate-measuring device body 1. The flow rate-measuring device body. 1 has the flow rate-measuring passage 11 for taking in and flowing a part of the fluid to be measured and the flow rate-detecting element 31 arranged in the flow rate-measuring passage 11. The flow rate-measuring passage 11 has an inlet 111 opening on convection faces opposite to the flow (the arrow A) of the foregoing fluid to be measured in the flow rate-measuring device body 1, and an outlet 112 opening on the rear side of the foregoing convection faces. An opening area (hereinafter referred to as cross section in a direction crossing the center axis of the flow rate-measuring passage 11 at right angles) of the flow rate-measuring passage 11 gradually decreases from the inlet 111 to the outlet 112.
The flow rate-measuring passage 11 shown in FIGS. 16 and 17 has a function of effectively decreasing uneven flows and eddies by contraction flow caused by the foregoing unique flow passage structure and by rectification of the fluid to be measured on the basis of the contraction flow described in detail in the specification of the prior application art, even if the fluid to be measured flowing into the flow rate-measuring passage 11 contains eddies or a flow having large unevenness in the flow velocity distribution (hereinafter referred to as drift) like a gas that has passed through a plugged honeycomb, for example.
The flow of the introduced fluid at the inlet 111 of the foregoing flow rate-measuring passage 11 is schematically illustrated in FIG. 17. That is, the fluid to be measured flowing in at the center of the flow rate-measuring passage 11 and in its vicinity as indicated by the arrow B flows toward the outlet 112 and is discharged from the outlet 112 to the outside of the flow rate-measuring passage 11. However, there exists a problem in that a part of the fluid to be measured which has flown in at ends of the inlet 111 flows backward and gets out of the inlet 111 as indicated by the arrow C. Such a backflow of a part of the fluid brings about a problem of turbulence in the flow at the inlet 111 of the flow rate-measuring passage 11 and deteriorates the rectification action based on the mentioned contraction flow due to the flow rate-measuring passage 11.
The present invention was made to resolve the above-discussed problems incidental to the foregoing prior application art, and has an object of providing a flow rate-measuring device capable of measuring a flow rate of a fluid containing any drift or eddy more accurately than the conventional measuring devices.
(1) A flow rate-measuring device according to the present invention comprises: a flow rate-measuring device body having a flow rate-measuring passage for measuring a flow rate of a fluid to be measured; a flow rate-detecting element disposed in the flow rate-measuring passage; and a leak flow passage using a part of an inlet of the flow rate-measuring passage as a leak flow passage inlet and allowing the fluid to be measured which has flown in from the leak flow passage inlet to leak out of the flow rate-measuring passage at a portion upstream from an outlet of the flow rate-measuring passage.
As a result of such construction, the fluid to be measured which has flown in at the ends of the inlet of the flow rate-measuring passage does not flow backward like the flow of the arrow C indicated in FIG. 17. But, the flow passing through the leak flow passage is discharged from its discharge port to the outside of the flow rate-measuring passage. Consequently, in the flow rate-measuring device according to the invention, a backflow occurred in the prior application art does not substantially take place, or at least the quantity of the backflow is decreased. This solves the problem of increase of turbulant flows caused by the backflow in the flow rate-measuring passage.
(2) It is preferable that an opening area in a direction crossing a center axis of the flow rate-measuring passage at right angles in the flow rate-measuring passage, at least in an upstream region communicating to the inlet of the flow rate-measuring passage, gradually decreases from the upstream to the downstream of the flow rate-measuring passage.
As a result of such construction, most of the fluid that has flown into the flow rate-measuring passage without being substantially influenced by the back flow flows toward the outlet of the flow rate-measuring passage. Meanwhile the flow is smoothly contracted according to the gradual decrease of the opening area. Therefore the flow rate-measuring device according to the invention effectively performs the rectification due to a contraction flow described in detail in the prior application art. Consequently it is possible to measure the flow rate of the fluid to be measured with less error without jointly using any rectifier even when the fluid contains any drift or eddy.
(3) A flow rate-measuring device according to the present invention comprises: a flow rate-measuring device body having a flow rate-measuring passage for measuring a flow rate of a fluid to be measured; a flow rate-detecting element arranged in the flow rate-measuring passage; and a leak flow passage using a part of an inlet of the flow rate-measuring passage as a leak flow passage inlet and allowing the fluid to be measured which has flown in from the leak flow passage inlet to leak out of the flow rate-measuring passage at a portion upstream from an outlet of the flow rate-measuring passage, wherein an opening area in a direction crossing a center, axis of the flow rate-measuring passage at right angles in the flow rate-measuring passage, at least in an upstream region communicating to the inlet of the flow rate-measuring passage, gradually decreases from the upstream to the downstream of the flow rate-measuring passage.
As a result of such construction, the same advantages as described in the foregoing (1) and (2) are obtained.
(4) It is preferable that the opening area of the inlet of the flow rate-measuring passage is 1.3 to 3 times as large as the opening area of the outlet of the flow rate-measuring passage.
As a result of such construction, the following advantages are obtained. That is, the fluid to be measured pulsates due to acceleration or deceleration of the engine, and this pulsation causes occurrence of eddy at the rear of the outlet of the flow rate-measuring passage. On the other hand, the air cleaner element is plugged more with the passage of time as it is used, and the plugged air cleaner element causes a drift in the fluid to be measured. When the opening area ratio of the inlet and the outlet of the flow rate-measuring passage remains within the above-mentioned range, it is possible to measure the flow rate withless error thereby solving both problems of the eddy and the drift.
(5) It is also preferable that the flow rate-measuring device body is a columnar body in which each of faces of the flow rate-measuring device body, where the inlet and the outlet of the flow rate-measuring passage are opened, is rectangular or almost rectangular.
As a result of such construction, the device exhibits an advantage of assisting the rectification performed by the flow rate-measuring passage by subdividing or fractionizing. the eddies contained in the fluid to be measured before flowing into the flow rate-measuring passage.
(6) It is also preferable that sides of the outlet of the flow rate-measuring passage extending in a direction of long sides of the face, where the outlet is opened, of the flow rate-measuring device body are longer than short sides of the face.
As a result of such construction, the following advantages are obtained. The fluid to be measured pulsates due to acceleration or deceleration of the engine, and this pulsation generates eddy at the rear of the outlet of the flow rate-measuring-passage as described above. This eddy may be pushed back to the outlet and block the outlet when the engine is decelerated. Since the sides extending in the direction of the long sides of the face, where the outlet is opened, of the flow rate-measuring device body are longer than the short sides of the face, the eddy cannot block the outlet. Consequently, even a big pulsation flow does not cause a problem of decrease in the flow velocity of the fluid to be measured in the vicinity of the flow rate-detecting element, and the flow rate can be measured with less error.
(7) It is also preferable that the flow rate-measuring device body is disposed in a duct of the fluid to be measured so that a center axis of the flow rate-measuring passage thereof may be substantially coincident to a center axis of the duct, and at least a part of convection faces opposed to the flow of the fluid to be measured of the flow rate-measuring device body is streamlined to reduce a flow resistance of the fluid to be measured.
As a result of such construction, the following advantages are obtained. When the convection faces are flat like the convection faces in the prior application art shown in FIGS. 16 and 17, a stagnation point is generated on the convection faces due to stemming of the fluid to be measured. Thus smooth flowing of the fluid into the flow rate-measuring passage is obstructed. However, when each of the convection faces is streamlined like a bow, the stemming and the stagnation point due to the stemming are not generated. Thus a steady flow is assured and the flow rate is measured with less error.
(8) It is also preferable that the inlet of the leak flow passage is located at an end portion of the inlet of the flow rate-measuring passage, and an outlet of the leak flow passage is located upstream from a portion where the flow rate-detecting element is disposed.
(9) It is also preferable that an opening area of the leak flow passage in a .direction crossing a center axis of the leak flow passage at right angles gradually decreases from the inlet to the outlet.
As a result of such construction, especially in the aspects of (1) or (3), when gradually decreasing the opening area of the leak flow passage in the direction crossing the center axis of the leak flow passage at right angles from the inlet to the outlet, the mentioned function of preventing back flow is more improved. At the same time, the rectification action due to a contraction flow in the flow rate-measuring passage is more improved.
(10) It is, also preferable that portions of the flow, rate-measuring device body in contact with the fluid to be measured are formed to be symmetrical on both sides of a face passing through the center axis of the flow rate-measuring passage and crossing a side wall of the flow rate-measuring device body at right angles.
As a result, unevenness disappears from a flow resistance at least in a direction of both sides of a column consisting of the flow rate-measuring device body, and the flow velocity distribution becomes uniform. Consequently, the quantity of the flow containing a drift is measured with less error.